Field
The present invention relates generally to column grid array (CGA) semiconductor packaging, and more particularly to an apparatus for aligning and depositing a plurality of electrical interconnect members, such as pins, solder columns, micro-coil springs, or other cylindrically shaped metallic parts in an array pattern on a ceramic or plastic substrate.
Description of the Related Art
Typically, an intermetallic connection is formed between a plurality of solder columns and the conductive pads on a land grid array (LGA) substrate. Initially, a layer of solder paste is applied to cover the array of conductive pads on the LGA. After heating, the solder paste reflows on the conductive pads causing an intermetallic connection between the solder columns and the conductive pads on the LGA. The LGA substrate material may consist of ceramic or plastic materials. After completion of the reflow process, the LGA with solder columns is known in the art as a column grid array (CGA) or ceramic column grid array (CCGA).
In general, the number of solder columns on a CGA device may range from 4 to 3000, or more, as the density of electronic devices and integrated circuit packages continues to increase.
An alternative to CGA column grid array devices is ball grid array (BGA) devices. BGA devices contain an array of solder spheres (balls) to provide electrical connections between the conductive pads on a BGA substrate and the printed circuit board (PCB). In the art, BGA substrates that are constructed of ceramic material (such as alumina or Al2O3) are known as a ceramic ball grid array (CBGA). Ceramic substrates are often required in harsh environments or when excessive heat and power is present.
However, one problem with BGA devices is that a substantial difference in the coefficient of thermal expansion (CTE) can exist between BGA substrates and the PCB board. The problem with CTE differences becomes more problematic when large size ceramic CBGA substrates are attached to PCB boards that are made of plastic glass-woven material such as FR-4, FR-5 or polyimide. Such differences in the coefficient of thermal expansion causes deformation of the solder spheres (solder balls) interconnecting a ceramic BGA device to a PCB board. Over time, the electrical connection between the solder ball and metal pad will break between large size ceramic BGA substrates and a plastic glass-woven PCB due to CTE mismatching issues.
The problem with CTE mismatch has been addressed by using cylindrical solder columns instead of solder spheres (solder balls) as the electrical interconnect between ceramic substrates and the plastic PCB boards. Taller cylindrically shaped solder columns are generally more compliant to better absorb CTE differential thermal expansion rates between the CGA and the PCB board. Wider solder columns are generally more structurally robust to support the load weight of heavy ceramic substrates. However, the maximum diameter of the solder column is normally constrained by the pitch (spacing) of the conductive pads on the CGA package as well as by the diameter of the conductive pads.
Traditionally, solder columns are cylindrically shaped and typically have a diameter of approximately 0.51 mm (0.020-inch) and a height of approximately 2.21 mm (0.087-inch). Solder columns may also be as small as 0.20 mm (0.008-inch) in diameter or more than 0.889 mm (0.035-inch) in diameter. Furthermore, the length of solder columns may be as short as 0.25 mm (0.010-inch) or as long as 3.81 mm (0.150-inch) or more.
The conductive pads on the LGA substrate are covered with a controlled thickness of solder paste before attaching a plurality of solder columns to the LGA substrate. Typically, solder paste consisting of low melting point tin-lead alloy, such as Sn63/Pb37, is preferred for applications within the fields of aerospace, military and defense industries. However, lead free solder paste alloys such as SAC305 (Sn96.5/Ag3.0/Cu0.5), or other Pb-free alloys, may be used for applications requiring lead-free materials.
Solder columns are typically made of high melting temperature solder such as Pb90/Sn10, Pb85/Sn15 or Pb80/Sn20. Solder columns may be wrapped with copper ribbon tape as disclosed in U.S. Pat. No. 4,664,309.
An alternative to solder columns is micro-coil springs that are typically made of beryllium copper (Be—Cu) alloy and electroplated with tin-lead solder (Sn60/Pb40) or other plating such as nickel-gold (Ni—Au) or Silver (Ag). Yet another alternative to solder columns are solid copper columns or other conductive materials.
Solder columns are generally vertically positioned perpendicularly onto a corresponding array of conductive pads on the LGA substrate. The substrate together with high temperature solder columns, or alternative pins and a layer of low temperature solder paste are then heated so that the solder paste is reflowed to make an intermetallic fillet connection between the solder columns and the LGA pads, without melting or damaging the solder columns. The completed package with attached solder columns is known in the art as a column grid array (CGA) or ceramic column grid array (CCGA) package.
A secondary procedure is required to mount the CGA package onto the PCB board. The process of connecting the CGA package to the PCB board requires the CGA to be reflowed again, without melting or collapsing the solder columns. A controlled layer of low temperature melting solder paste is applied to a corresponding plurality of contact pads on the PCB board. The CGA package is placed onto the solder paste covered pads on the PCB board. The PCB board along with one or a plurality of CGA packages (as well as other components) is heated and reflowed resulting in an intermetallic fillet that holds the CGA solder columns to the PCB board.
In the prior art, various methods and apparatuses have been utilized to mount cylindrically shaped solder columns into an array pattern by hand using tweezers or via vibration or with a vacuum pick-up tool.
In the prior art, methods using tweezers to place solder columns by hand are time consuming and require an operator with dexterity to perform many repeated steps. For example, it may require roughly one-hour to load 900 solder columns onto a CGA substrate by hand, assuming that a person using tweezers is able to pick-up, transfer and place one solder column every four seconds. In addition, in the prior art, a person using hand placement methods may result in errors as the operator often fails to complete the specified pattern.
In the prior art, methods to deliver interconnect members use vibration (e.g., require an inclined vibration machine with elongated alignment and a vacuum pick-up tool to position solder pins onto a CGA substrate package) or a sacrificial adhesive tacky tape layer in a carrier plate to retain and position an array of solder pins onto a CGA substrate package.